I need to gain weight

Jamie
So when my life gets stressful and I'm filled with a lot of anxiety, I end up skipping meals, or smoking a cigarette on my lunch break (rather than eating), or just forgetting to eat a meal all together. and with work driving me insane and my personal life stressing me out, recently I've lost a lot of weight.
I'm a tiny girl to begin with, 5'4" and usually around 100lbs. I just can't pack on weight and keep it! 
I've been drinking ensure once a day to get protein but I don't know what else to do because I can't afford to go on some crazy expensive organic diet or anything. I'd love to just hit the gym and gain muscle weight but after working the hours I do, it's nearly impossible
does anyone have tips?